1. Field of the Invention
The present invention relates to the field of demodulation of signals. In its preferred embodiment, the present invention has application in communication devices such as pagers.
2. Description of the Related Art
Pagers have gained popularity in recent years. Several key issues in the design of pagers are cost, size and power consumption. Very briefly, a pager receives radio frequency (RF) signals, converts them to digital signals, and displays resulting messages or takes other action (such as control of functions of the pager) based on the received signals. The conversion process from RF signals to digital signals requires several steps including converting the RF signal to a baseband and demodulation of the signal to digital representations of the signal. Of course, the conversion approach will vary dependent on the selection of a particular digital modulation technique.
Typical prior art approaches to conversion of the RF signal employ discrete analog circuits such as filters. Such circuits are both relatively expensive and consume significant power. An alternative prior art approach is illustrated by FIG. 1. In the circuit of FIG. 1, the FSK modulated input signal is demodulated by converting the input signal at a carrier frequency fi to baseband by mixing (using mixers 106, 107) the input signal with a local oscillator signal (from oscillator 110) having a center frequency equal to the center frequency of the modulated signal. This results in two baseband signals Ia and Qa which have a quadrature phase relationship. The resulting mixed signals are processed by comparators 112, 113, respectively to provide I and Q signal to digital phase detector 114. The phase difference between the I and Q signals may be plus or minus 90 degrees and is a function of the instantaneous frequency of the FSK modulated inputted signal. Therefore, for a positive frequency deviation, the phase difference has one sign and for a negative phase deviation, the phase difference has the opposite sign. Phase detector 114 detects the relative phase by measuring the sign of the phase deviation. It is noted that comparators 112, 113 allow use of the digital phase detector 114 removing the requirement for some analog circuitry. However, this circuit is limited in the data rate that it can handle because the information obtained from the baseband signals is limited to the determination of the time when the signal crosses zero value. Therefore, for a pair of baseband signals (I, Q), only four points (zero crossings) per cycle can be determined. As the data rate increases (or as the deviation frequency is reduced), there are not sufficient zero crossings per symbol period to provide enough information for the demodulation process. Also, the architecture in FIG. 1 is limited to demodulation of binary FSK modulation. For higher order FSK modulations, the comparators can be replaced by A/D converters; however, this complicates the digital phase detection and A/D circuitry which may lead to power consumptions too high for use in portable devices such as pagers.
Therefore, what is needed is a circuit and method which provides for efficient demodulation of a modulated signal which is transmitting data at data rates which are high relative to the frequency deviation of the modulated FSK signal.